VLA Test/Observation Coordination Meeting B.G. Clark January 16, 2003 1. eVLA first local oscillator B. Clark says that the eVLA LO system is being redesigned. The design being considered has very large steps in the frequencies settable in the first local oscillator. The synthesizer is settable in steps of 256 MHz. This synthesizer is doubled (for K band) or tripled (for KA and Q bands) before being used as the first local oscillator, giving first LO steps of 512 and 768 MHz, respectively. The first LO is used to convert an approximately 8 GHz IF bandwidth, which can thus be positioned only in 512 MHz or 768 MHz steps, with a modest amount of wiggle provided by the second LO. BC believes this step size does not restrict being able to set up on any two spectral lines, but may make impossible some setups attempting to setup on three lines. There may also be a problem in continuum if there are two strong interferers spaced a little more than 4 GHz apart; it may not be possible to nestle a 4 GHz IF band between the. But he advocates the new design as significantly more robust, feeling the disadvantages above are minor. 2. The subreflector rotation trick We have long rotated the subreflectors at Q band as a partial compensation for the aberration caused by the feed leg sag. K. Sowinski reports that this is being implemented at K band as well. The new K band receivers are located at a point on the feed ring where this is profitable. The five or so antennas that do not yet have the new K band receivers are unaffected. In the course of implementing this, he found a few antennas for which the rotation was improperly set, and for those few antennas there is considerable improvement in gain. For the others, there is a modest improvement, depending on elevation. B. Butler points out that this will radically modify the K band gain curves, and that observers must be notified about this. It is noted that a gain-curve/flux-scale determination run is scheduled for early February.